cgo: silence warning for C call returning const pointer

Fix suggested by Albert Strasheim.

R=adg
CC=golang-dev
https://golang.org/cl/2154041

src/cmd/cgo/out.go

@@ -107,7 +107,11 @@ func (p *Package) structType(n *Name) (string, int64) {
 			fmt.Fprintf(&buf, "\t\tchar __pad%d[%d];\n", off, pad)
 			off += pad
 		}
-		fmt.Fprintf(&buf, "\t\t%s r;\n", t.C)
+		qual := ""
+		if t.C[len(t.C)-1] == '*' {
+			qual = "const "
+		}
+		fmt.Fprintf(&buf, "\t\t%s%s r;\n", qual, t.C)
 		off += t.Size
 	}
 	if off%p.PtrSize != 0 {